We are in a time of crisis caused by the bark beetle calamity. The causes are many, from the planting of spruce monocultures in localities where they do not belong, to the neglected care of infested trees to the extreme temperatures and droughts that have plagued our forests in recent years and are helping to reproduce pests. Questions about why forests are so vulnerable, what is harmful to them, the conditions under which they can defend themselves, and many others are being addressed by countless scientific teams. One of them, working at the Faculty of Forestry and Wood Sciences, focused on whether mixed forests have the potential to replace monocultural ones.
White fir and beech are considered to be the most likely species that could replace monoculture stands of Norway spruce across the European continent. The use of beech in the transformation of these stands has been increasing recently and fir is probably the most suitable tree for the future European climate, as it is accustomed from the ancient past (Middle Holocene) to warmer climates than we have today. It is already clear that Norway spruce, planted since the middle of the 19th century, is increasingly suffering from frequent summer droughts, strong storms and subsequent bark beetle raids.
A study on the impact of environmental changes on the growth of beech and white fir is part of the key question of whether spruce monocultures can be replaced by more suitable tree species. By analysing a series of samples of beech and fir annual rings from 17 sites selected across Europe, the researchers monitored the development of radial growth of these trees. They compared its dependence on geographical location, on the status of the canopy tree (whether it was a dominant, codominant, subdominant or suppressed tree), whether the forest is managed or not and whether the growth of co-growing firs and beeches was equally affected by pollution in time period from 1970s to 1990s and by warming over the last decades. And why beech and fir? Recent studies have confirmed that these trees can achieve higher stem growth in mixed stands. So, what is their ecological potential?
We will first look at the results of the analysis of annual ring samples. They told us that beech growth accelerated between the 1950s and 1980s, especially in the northern areas of its occurrence, followed by a slow decline. On the contrary, in the southern habitats there was a slowdown or a decrease in growth. As for the fir, a decrease in the growth rate was observed in the same period in most of the examined localities. The main cause was sulphur air pollution, which devastated fir stands throughout Europe. This decline subsequently turned into a sharp increase in the northern areas and, conversely, a further decrease in the south.
The statistical processing of the data showed that the social position of the tree, i.e. whether it was, for example, dominant, did not affect the thickness of annual rings. Likewise, no differences in tree growth were found between commercial and non-intervention forests. In contrast, environmental conditions have proven to be important factors. Drought had the most significant effect on the growth of fir in the southern localities. This factor was best resisted by the Balkan populations, probably due to their higher genetic and functional diversity. The positive effect of rising summer temperatures in the more northern areas as well as higher altitudes was seen in beech. Based on this result, a further acceleration of beech growth could be expected, as climate warming is unlikely to end there. Nevertheless, it is necessary to take into account regional differences associated with the local climate and habitat productivity.
In general, higher species diversity can help mitigate the effects of short-term climate events, such as drought, through better resilience at the level of forest ecosystems, but may not protect mixed forests from the long-term effects of climate change. What does this mean for spruce monocultures? Probably nothing dramatic. In the long run, even mixed stands do not necessarily mean salvation for our forests. So good luck to the Forest, ruin to the bark beetles!
Bosela, M. et al. (2018). Contrasting effects of environmental change on the radial growth of co-occurring beech and fir trees across Europe. Science of the Total Environment, vol. 615, pp. 1460-1469.
prof. Ing. Miroslav Svoboda, Ph.D.
Professor Svoboda is the head of the Department of Forest Ecology and a member of the Scientific Board of FLD and CULS. At the same time, he also teaches at the Faculty of the Environment. The collection of topics he deals with as a scientist is really wide, from disturbances and their influence on forest stands, through analyses of the dynamics of spruce forests to the evaluation of rotting wood.
Prepared by: Lucie Hambálková